The correct order of thermal stability of hydrides of group `15` is

Options

A. `NH_(3) gt PH_(3) gt AsH_(3)`
B. `NH_(3) lt PH_(3) lt AsH_(3)`
C. `NH_(3) gt PH_(3) lt AsH_(3)`
D. `NH_(3) lt PH_(3) gt AsH_(3)`

Answer

To determine the correct order of thermal stability of hydrides of group 15 elements (nitrogen, phosphorus, arsenic, and so on), we need to analyze the factors that influence thermal stability, primarily focusing on bond strength and bond dissociation enthalpy.

### Step-by-Step Solution:

1. **Identify the Group 15 Elements**: The elements in group 15 include nitrogen (N), phosphorus (P), arsenic (As), antimony (Sb), and bismuth (Bi). However, we will primarily focus on the first three: NH₃ (ammonia), PH₃ (phosphine), and AsH₃ (arsine).

2. **Understand Bond Strength**: The thermal stability of hydrides is directly related to the bond strength between the central atom and hydrogen. Stronger bonds are harder to break, leading to greater thermal stability.

3. **Consider Atomic Size**: As we move down the group from nitrogen to phosphorus to arsenic, the size of the central atom increases. This increase in atomic size leads to a decrease in bond strength because the larger atom has a longer bond length with hydrogen, resulting in weaker interactions.

4. **Analyze Bond Dissociation Energy**: The bond dissociation energy (BDE) is a measure of the strength of a chemical bond. As the size of the central atom increases, the bond dissociation energy decreases. Therefore, we can conclude that:
- NH₃ has the highest bond strength and bond dissociation energy.
- PH₃ has lower bond strength and bond dissociation energy than NH₃.
- AsH₃ has the lowest bond strength and bond dissociation energy among the three.

5. **Establish the Order of Thermal Stability**: Since thermal stability decreases with decreasing bond strength, we can establish the order of thermal stability of the hydrides:
- NH₃ > PH₃ > AsH₃

6. **Final Conclusion**: The correct order of thermal stability of hydrides of group 15 is:
- **NH₃ > PH₃ > AsH₃**